A benchmark is a statistical point of interest used for comparison. They are usually expressed as an average across an industry, country, or type of advertising (or all three combined). These averages (or benchmarks) are then used as goals to beat.
For example, if you had a website about helicopters, you might want to make sure that your adverts outperformed other vehicle companies.
You would therefore look up the average CTR (or CPC or CPA, depending on your goals) for the vehicle industry, and use that figure as your benchmark to improve upon.